> > "Hversu margt hefir hér fyrirmanna látist?" (...)
> > Flosi svarar: "Hér hefir látist Njáll og Bergþóra og synir þeirra

allir,..."

Gael asked:

> Why is the singular used: hefir látist instead of hafa látist ?

Because 'margt' is the subject. It's just a quirk of Old Norse grammar
that, in certain situations, a neuter singular quantifier can be used
like this with genitive plural of the noun that is quantified.

margt manna = margir menn "many men/people"
fátt manna = fáir menn "few men/people"

The dictionary entries for 'margr' and 'fár' have more examples and
details. A similar, frequently used expression is 'hvat manna ertu'
"who are you?"